Nobility is a traditional hereditary status. In the feudal system, the nobility are generally those who hold a fief, often land and/or office, under vassalage, i.e. in exchange for allegiance and various, mainly military, services to the Monarch and at lower levels to another nobleman.
All nations of the Westlands follow the classic model of feudal nobility. By contrast, the Aiel live by the system of ji'e'toh, whereas the Seanchan adhere to a much harsher system of blood right and ownership.